1. Field of the Invention
The present invention relates to footwear. The invention concerns, more particularly, an article of footwear having a removable foot-supporting member.
2. Description of Background Art
One objective of modem athletic footwear design is to minimize weight while maximizing comfort, stability, and durability. In order to meet this goal, designers utilize a broad range of materials, shoe components, and shoe-making methods. The basic design of conventional athletic footwear, however, remains largely uniform and includes two primary elements, an upper and a sole structure. The upper may be formed of leather, synthetic materials, or a combination thereof and comfortably receives the foot while providing ventilation and protection from the elements. The sole structure includes multiple layers that are conventionally referred to as an insole, midsole, and outsole. The insole is a thin, padded member located adjacent to the foot that improves overall comfort of the footwear. In many articles of footwear, the insole is removable and may be replaced. The midsole forms the middle layer of the sole and often incorporate a resilient foam material, such as polyurethane or ethyl vinyl acetate, that attenuates shock and absorbs energy when the footwear is compressed against the ground. Unlike the insole, midsoles are integrally-formed with the footwear and may not be replaced or modified by a wearer. The outsole is fashioned from a durable, wear resistant material, such as carbon-black rubber compound, and typically includes a textured lower surface to improve traction. A disadvantage relating to the laminar design of conventional sole structures is that the overall flexibility of the sole structures are decreased, particularly in the forefoot.
Some modern footwear designs depart from conventional designs by replacing a majority of the midsole with a removable sockliner. Footwear of this type includes an upper, a sockliner, a thin midsole, and an outsole. The sockliner, therefore, functions as the primary shock attenuation and energy absorbing element in both the heel and forefoot regions of the footwear. Although this design provides greater flexibility in the forefoot area than conventional laminar designs, the relatively large thickness of the sockliner in the heel region may cause chafing or blisters due to movement of the foot in relation to the upper.
An important aspect of footwear design involves controlling the motion of the foot during activities that involve running. For many individuals, the motion of the foot while running proceeds as follows: The heel strikes the ground first, followed by the ball of the foot. As the heel leaves the ground, the foot rolls forward such that the toes make contact, and finally the entire foot leaves the ground to begin another cycle. During the time that the foot is in contact with the ground and rolling forward, it also rolls from the lateral side to the medial side, a process called pronation. That is, normally, the outside of the heel strikes first and the toes on the inside of the foot leave the ground last. While the foot is air borne and preparing for another cycle, the opposite process, called supination, occurs. Pronation is a normal and beneficial aspect of running, but may be a potential source of foot and leg injury, particularly if it is excessive.
Footwear designed for individuals with excessive pronation often incorporate pronation control devices to limit the degree of pronation during running. In general, pronation control devices are an additional element, such as a heel counter, or a modification of an existing element, such as the sole structure. In general, a heel counter is a rigid member that extends around the heel portion of the footwear, thereby limiting movement of the heel. Additional support may be provided to a heel counter by including a bead of material, as disclosed in U.S. Pat. No. 4,354,318 to Frederick, et al. Another prior art technique that enhances pronation control following foot impact involves building up the heel counter, as disclosed in U.S. Pat. Nos. 4,255,877 to Bowerman and U.S. Pat. No. 4,287,675 to Norton, et al.
The sole structure may also be modified to control pronation. For example, the medial side of the sole structure may include higher density cushioning materials, as disclosed in U.S. Pat. Nos. 4,364,188 to Turner, et al. and U.S. Pat. No. 4,364,189 to Bates. Similarly, a less compressible fluid chamber may be incorporated into the medial heel area of the sole structure, as disclosed in U.S. Pat. Nos. 4,297,797 and 4,445,283, both to Meyers. Another prior art technique, as disclosed in U.S. Pat. No. 5,247,742 to Kilgore, et al., involves incorporating a compression resistance increasing member into the midsole.
Although the prior art pronation control techniques exhibit a degree of success in controlling pronation, the techniques also add to the weight and manufacturing expense of footwear. The present invention was designed to cooperatively utilize a combination of structural features in a manner that effectively reduces the disadvantages of prior art sole structures.